“The investigation team has examined the plane’s status, sealed the relevant flight data and information, and talked to the crew members. The (CAAC) Northeast Regional Administration has sent the flight data recorder and the cockpit voice recorder to the China Academy of Civil Aviation Science and Technology for analysis. Further investigation will follow,” a CAAC statement said.

Oxygen masks dropped from the ceiling of the Boeing 737 that was carrying 153 passengers and nine crew on board, descending to 10,000 feet, though the plane later climbed back to 26,000 feet and arrived safely at its destination.

There were no reported injuries and the aircraft was not damaged, the aviation regulator said.

The CAAC said the emergency descent was triggered by a cockpit altitude warning.

Air China said on Wednesday an investigation was focused on pilots following claims crew had been smoking in the cockpit.

“If the investigation reveals the crew violated regulations, the company will deal with those responsible with zero tolerance,” Air China said.

The plane descended at 7.40pm between Shantou and Xiamen, half an hour after it took off from Hong Kong International Airport.

Experts said the subsequent actions of the pilot, not diverting to the nearest airport, also raised questions for the investigation.

David Newbery, president of the Hong Kong Airline Pilots Association and a Cathay Pacific Airways pilot, said: “If the oxygen masks had been deployed, it would be foolhardy to [continue flying] because there would be no oxygen left in the event of a subsequent depressurisation.”

One area that is likely to be examined is how the pilots handled the plane’s ventilation system.

A Cathay Pacific aircraft engineer, who declined to be named, explained that pilots wishing to conceal cigarette smoke could turn off a fan in the cockpit to prevent smoke circulating in the cabin.

“However, the shut down of the fan would not have caused the cabin altitude warning message.”

The engineer added that it was possible the pilots flipped the wrong switch, such as closing the pneumatic isolation valves, affecting the pressure in the cabin causing the masks to drop.